The exhaust system of a vehicle carries hot exhaust gases from the engine to a location on the vehicle where the gases may be safely and efficiently emitted. The engine typically is in the front of the vehicle, while the tail pipe typically terminates at or near the rear of the vehicle. The exhaust system must assume a very tortuous alignment along its length to avoid other structural components of the vehicle such as the passenger compartment, the rear axle, the fuel tank, the trunk, tire wells and such. An exhaust system is likely to include: a manifold which is mounted to the engine and which collects the hot exahust gases from several cylinders; an exhaust pipe which extends from the manifold; one or more mufflers connected to the exhaust pipe and operative to reduce the noises produced by the hot flowing exhaust gases; and a tail pipe extending from the muffler to the rear of the vehicle. Most vehicles also include a catalytic converter which is operative to chemically convert certain noxious gases into a less objectionable form.
In many instances, components of the exhaust system will pass in proximity to parts of the vehicle that may be sensitive to the heat generated by the exhaust system. In other situations, heated components of the exhaust system may be disposed where a potentially dangerous contact with objects external to the vehicle is possible.
In recent years, vehicular manufacturers have changed the engine and the exhaust system to effect more complete combustion of fuels and thereby to reduce certain pollutants. These engine operating changes typicially are carried out by directing additional air into the engine or manifold. Although these engineering techniques are successful, they tend to significantly increase the temperature of exhaust gases with corresponding increases in the temperatures of the various exhaust system components. In certain situations, the hot exhaust pipes or tail pipes have been known to structurally damage adjacent parts of a vehicle. In other situations, exhaust system components have created a potential for fire in leaves or other debris that may directly contact an exhaust system component. Furthermore, the exhaust system components retain their heat for a considerable period of time and can burn people working on or near the vehicle.
Heat shields often are employed to separate a hot section of an exhaust system from an adjacent area that may be particularly sensitive to heat. The typical prior art heat shield has been stamp formed into a shape corresponding to the shape of the section of the exhaust system to be isolated. The heat shield then may be clamped or welded to a section of the exhaust system such that a gap is formed between the exhaust system and at least a portion of the heat shield. The gap between the exhaust system and the heat shield does not efficiently conduct heat. Consequently, the heat shield will be at a considerably lower temperature. Furthermore, the heat shield prevents physical contact with the adjacent hot portion of the exhaust system. One particularly effective heat shield is shown in U.S. Pat. No. 4,478,310 which issued to Donald P. Harter on Oct. 23, 1984, and which is assigned to the assignee of the subject invention. Although the heat shield shown in U.S. Pat. No. 4,478,310 is both effective and inexpensive, it cannot be adapted for the nonlinear tubular sections of an exhaust pipe or tail pipe. The prior art heat shields formed for such nonlinear tubular sections have been stamp formed and have covered very short sections of the exhaust system. Stamp formed heat shields require the initial costly investment in the dies to be used in forming the member. Because of these initial investments, stamp formed heat shields are extremely costly, and are virtually cost prohibitive on all but very large orders. Furthermore, even on extremely large orders, the shapes that are possible with the stamp forming equipment and processes are limited. In applications where a heat shield is required over a long circuitous section of an exhaust pipe or tail pipe, it has been found impractical to form a single nonlinear heat shield. Rather, the prior art typically would employ several short stamp formed heat shield sections mounted in series along a circuitous exhaust system component. This stamp forming has been found to be especially impractical where protection from heat must be applied to alternating sides of the exhaust pipe or tail pipe or where sections require protection on all sides.
Certain extemely hot tubular exhaust system components require protection on all sides along a considerable length of the exhaust system. In these situations, air gap pipes have been employed which comprise inner and outer tubular members with a generally annular air gap therebetween. A particularly effective air gap pipe and an efficient method for producing such a pipe is disclosed in U.S. Pat. No. 4,501,302 which issued to Jon W. Harwood on Feb. 26, 1985, and which is assigned to the assignee of the subject invention. The manufacturing method described in U.S. Pat. No. 4,501,302 involves the bending of selected inner and outer pipes into substantially identical nonlinear configurations. Appropriate supports are formed in or are attached to the inner or outer pipe to enable the desired radial separation therebetween. The outer pipe then is cut longitudinally in half by a programmed cutting apparatus. More particularly, the cutting apparatus is programmed to follow the precise circuitous shape of the outer pipe. The outer pipe halves then are separated and the inner pipe is positioned therebetween. The outer pipe halves may then be appropriately reconnected to achieve an air gap pipe. The air gap pipe, the method of producing the air gap pipe and an apparatus for performing the method are further described in co-pending U.S. patent application Ser. No. 790,737 which was filed on Oct. 24, 1985, and which is entitled "METHOD AND APPARATUS FOR FORMING AN AIR GAP PIPE." The disclosures of U.S. Pat. No. 4,501,302 and of application Ser. No. 790,737 are incorporated herein by reference.
Despite the many advantages of the air gap pipe described in the above identified references, the prior art has not included a technique for efficiently, quickly, and inexpensively manufacturing heat shields, and particularly heat shields for nonlinear sections of exhaust pipes or tail pipes. Although the air gap pipe shown in U.S. Pat. No. 4,501,302 provides exceptional protection from heat, it often is necessary to protect only one side of the exhaust or tail pipe along certain sections of its length. Thus, the air gap pipe would utilize substantially twice as much material as might be required for these sections. Furthermore, on other sections of an exhaust or tail pipe, an air gap pipe may be less desirable than a heat shield. Specifically, it may be preferable to provide a heat shield on one side of a pipe while allowing a continuous flow of cooling air adjacent the other side. In still other situations, it may be desirable to mix heat shields with an air gap construction at various sections along the length of an exhaust system. Furthermore, it may be desirable to alternate the sides on which the heat shield is located in accordance with the vehicle specifications.
In view of the above, it is an object of the subject invention to provide an efficient method for producing a heat shield for a tubular member of a vehicular exhaust system.
It is another object of the subject invention to provide an efficient method for producing a nonlinear heat shield for a tubular member.
Another object of the subject invention is to provide a method for producing a hybrid exhaust system having both heat shields and air gap pipes along selected portions of the exhaust system.
Still another object of the subject invention is to provide a heat shield of unitary construction extending along substantially the entire length of a nonlinear tubular member.
A further object of the subject invention is to provide a method for producing a heat shielded tubular member that can accommodate differential expansion.